Flotation, and in particular, froth flotation is a physiochemical mineral concentration method that uses the natural and/or created differences in the hydrophobicity of the minerals to be separated. To enhance existing or to create new water repellancies on the surface of the minerals, certain heteropolar or nonpolar chemicals called collectors are added to the process. These reagents are designed to selectively attach to one or more of the minerals to be separated, forming a hydrophobic monolayer on their surfaces. This form makes the minerals more likely to attach to air bubbles upon collision. The combined air bubble/mineral particle mass is less dense than the displaced mass of the pulp, causing it to float to the surface, where they form a mineral-laden froth that can be skimmed off from the flotation unit, while the other minerals remain submerged in the pulp. The flotation of minerals with a negative surface charge, such as silica, silicates, feldspar, mica, clays, chrysocola, potash and others, from a pulp is achieved using cationic collectors. In iron and phosphate beneficiation processes, the impurities are floated away, leaving the valuable component behind. This process is called "reverse flotation". Cationic collectors are organic molecules that have a positive charge when in an aqueous environment. All cationic collectors have a nitrogen group with unpaired electrons present.
Three main categories of cationic collectors have found commercial application: fatty amines, ether amines and amine condensates. The fatty amines may be mono-functional or difunctional and the amine functionality may be primary, secondary or tertiary. Similarly, the ether amines may be primary amines or may be difunctional. An example of a condensate includes compounds such as RCONHCH.sub.2 CH.sub.2 NHCH.sub.2 CH.sub.2 NHCOR, and the like where R may be a straight or branched alkyl group of 6 to 22 carbon atoms.
In addition to the above-described amines, alkoxylated quaternary ammonium compounds and their salts have also been evaluated as cationic collectors.
These reagents may be applied either in neat form, particularly the ether amines and diamines, which are liquid at room temperature. The collectors may also be added in aqueous solution as the acid salt form.
Fatty amines are the product of ammonolysis of natural fats. This reaction produces primary amines with the carbon chain length associated with the various naturally occurring fats. The natural fats are essentially straight chain carbon linkages with varying degrees of unsaturation. The primary amines of chain length 16 and longer are poor surfactants and usually frothers must be added to make the process feasible. Some industries, such as those recovering phosphate and feldspar use custom-blended fatty primary amines with frothers and, occasionally, even emulsifying surfactants. This type of reagent not only incorporates the frothing characteristics of the frother, but also is a liquid product at lower temperatures, making it easier to handle.
If the fatty primary amine is reacted with acrylonitrile (CH.sub.2 .dbd.CH--C.tbd.N), the fatty product is a fatty diamine. The presence of the second nitrogen group provides the diamine with added surfactancy, making the use of frothers unnecessary. Fatty secondary amines can be produced either as a reaction product of fatty alcohols and ammonia with the presence of a catalyst, or as a hydrogen reduction product of fatty primary amines with a catalyst. The reaction of additional fatty alcohol with the secondary amines using a catalyst produces fatty trialkyl tertiary amine.
If an alcohol is reacted with acrylonitrile, the result is an amine with an oxygen atom in the chain three carbons from the nitrogen. The presence of the oxygen atom (ether linkage) imparts a hydrophilic character to the otherwise hydrophobic chain. This results in an amine with more solubility and somewhat weaker collecting properties than the fatty amines. A second contact of the ether amine with acrylonitrile forms an ether diamine.
An amine condensate is the product of the reaction of a polyamine with an organic acid. The polyamines are generally short chain length compounds with three or more nitrogen atoms in the chain. The organic acid is usually, due to its favorable economics, tall oil.
A number of patents are known in this art. For example, Canadian Patent No. 796,803 describes a froth flotation process for separating silica from an ore, which concerns frothing the ore in the presence of an aqueous medium containing an acid salt of a primary aliphatic ether amine having the general formula R--O--R'--NH.sub.2, where R is an aliphatic radical having 6 to 22 carbon atoms and R' is an alkylene radical having 2 to 6 carbon atoms. It is noted that the ether amines may be prepared by known methods of cyanoethylation (defined in chemical dictionaries as providing a --OCH.sub.2 CH.sub.2 CN group by reaction with acrylonitrile) of a primary aliphatic alcohol, or mixtures of such alcohols, including oxo alcohols, to prepare the corresponding ether nitriles and then hydrogenating the latter to prepare the corresponding ether amines. If the cyanoethylation uses acrylonitrile, then R' must be --CH.sub.2 CH.sub.2 CH.sub.2 --, propyl, as is indeed the case for nearly all of the amines listed in this patent.
A froth flotation process for separating silica from an ore, which involves frothing the ore in the presence of an aqueous medium containing a water dispersable acid salt of an aliphatic ether diamine having the general formula EQU R--O--CH.sub.2 CH(R")CH.sub.2 NHCH.sub.2 CH(R")CH.sub.2 --NH.sub.2
where R is an aliphatic radical having 1-13 carbon atoms, and R" is a hydrogen atom or a methyl group and floating off the silica from the ore, is set out in U.S. Pat. No. 3,363,758 (which corresponds to Canadian Patent No. 839,775). The ether diamines of this patent are prepared by reacting an aliphatic ether primary amine with acrylonitrile or methacrylonitrile and then hydrogenating the resulting aliphatic ether amine nitrile to produce the ether diamine.
U.S. Pat. No. 4,319,987 describes the use of primary aliphatic ether amines as silica collectors in the concentration of minerals by the froth flotation process. More specifically, the use of mixtures of primary methyl branched aliphatic ether amines and the partially-neutralized salts thereof as flotation reagents is presented. In a further aspect, the use of mixtures of 3-isooctoxypropyl monoamine and 3-isodecoxypropyl monoamine and/or the partially-neutralized acetate salts thereof as collectors for silica in the beneficiation of oxidized taconite ores is mentioned. The patent teaches that the mixtures of methyl-branched alkyl ether amine acetates are prepared from the corresponding methyl-branched, preferably oxo, alcohols or mixtures of alcohols by the "well-known" cyanoethylation reaction, subsequent catalytic reduction, and neutralization with the conjugate acid of the desired anion. As noted previously, cyanoethylation requires acrylonitrile as a coreactant.
Acrylonitrile, used during the manufacture of the diamines and etheramines of the collectors described above, is extremely poisonous, making it dangerous for the workers during the synthesis of the collectors. Further, any residual, nonconverted acrylonitrile can be harmful to the environment, especially to the fish that come in contact with the waste streams of the beneficiation plants. Certain iron flotation plants in Canada have been closed due to the possibility of decimating their fish industry in the region.
It would thus be desirable if effective alkyloxyalkaneamine collectors could be developed which are free of acrylonitrile, but with no loss of activity.